Combustion-related isoprene contributes substantially to the formation of wintertime secondary organic aerosols.

Isoprene is a key reactive organic gas involved in organic aerosol formation. While biogenic isoprene from terrestrial plants has been extensively studied and is recognized as a major contributor to secondary organic aerosol (SOA), high levels of observed SOA, especially in winter, cannot be fully explained by biogenic isoprene alone. In this study, we developed a comprehensive bottom-up emission inventory for isoprene, incorporating both biogenic and combustion sources and modeling their contributions to SOA in China from 2000 to 2016. Combustion-related isoprene emissions from open biomass burning and residential fuel combustion were estimated at 52.0 (39.1-65.7) Gg in 2000, declining to 14.8 (10.6-19.0) Gg by 2016. Open biomass burning contributes ∼40% of combustion-related isoprene emissions. Though, annually, combustion-related isoprene emissions were much smaller than the biogenic emissions, they did account for 32%-80% of total isoprene emissions in many north and west provinces in the colder months in 2016, and were even higher during the early 2000s owing to more biofuel-burning emissions. Model simulation results indicated that combustion-related isoprene could contribute 25%-40% of winter SOA in northern regions. Wintertime isoprene-derived SOA levels declined since 2000, corresponding with decreased combustion-related isoprene emissions; however, the extent of this decline varied regionally due to the influence of other precursors like nitrogen oxides (NOx). In the northeast region with high NOx levels, while combustion-related isoprene emissions decreased by >80% from 2000 to 2016, isoprene-derived SOA declined by only ∼20%. These findings highlight the previously underappreciated contributions of combustion-related isoprene to observed high wintertime isoprene-derived SOA levels.